The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the acquisition of data from the separate x-ray detectors in 2D detector arrays.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the"imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon a row, or one-dimensional array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In a volumetric CT system the fan beam also fans out along the z-axis and the detectors are arranged in a 2D array to acquire attenuation measurements in a plurality of slices disposed along the z axis. In some applications, such as lung imaging, high resolution is required in the slice direction and this requires that the dimension along the z-axis of each x-ray detector be very small. Thus, the output signal from each detector can be critically small, particularly in applications where the x-ray beam is highly attenuated. The resulting reduced signal-to-noise ratio can significantly reduce image quality. One aspect of this "low-signal problem" can be solved with more stringent noise requirements on the preamplifiers used in the Data Acquisition System ("DAS") and another aspect can be solved by using more efficient x-ray detector technology. But both of these solutions add considerable expense to the system. The problem can also be solved by increasing the x-ray dose, but this is not a desirable solution considering the increased radiation to the patient.